Methods and Equipment for Fault Tolerant IP Service

ABSTRACT

An Internet Protocol (IP) terminal, comprises communication means for communicating via an IP network, a processor and memory. The memory contains an operating software for the IP terminal and the processor is configured to execute the operating software. The operating software comprises a normal mode logic for implementing a normal mode operation and a restricted mode logic for implementing a restricted mode operation. The normal mode logic comprises program code for initiating a call of a first type under control of instructions from one or more dedicated servers. The restricted mode logic comprises program code for collecting connection information of other IP terminals and for initiating a call of a second type without instructions from the one or more dedicated servers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase application of InternationalApplication No. PCT/FI/2008/050252, filed May 8, 2008, which isincorporated herein by reference.

BACKGROUND

1. Field

The invention relates to methods, equipment and software products forfault tolerant Internet Protocol (IP) service.

2. Description of the Related Art

Voice over Internet Protocol (VoIP) service relates to transmission ofvoice traffic in data packets in a network supporting Internet Protocol(IP). When the present invention is made, state of the art in VoIPservice was defined in Internet standards RFC3550: A Transport Protocolfor Real-Time Applications (“RTP”); RFC2543, RFC3261: A SessionInitiation Protocol (“SIP”); RFC2327: Session Description Protocol(“SDP”); RFC3264: An Offer/Answer Model with the Session DescriptionProtocol (“SDP”); RFC3515: The Session Initiation Protocol (“SIP”) ReferMethod; and RFC 3265: Session Initiation Protocol (“SIP”)-Specific EventNotification. It is to be noted, however, the VoIP development is anongoing process, and the invention is equally applicable to future VoIPdevelopments.

A key element in VoIP service is Session Initiation Protocol Server, orSIP server. The SIP protocol operates with user agents and user agentservers. The server's task is to provide name-to-address resolution andlocation management in respect of the user. Other services which areessential to full operation of VoIP service network include Dynamic HostConfiguration Protocol (DHCP) which is a client-server networkingprotocol. A DHCP server provides configuration parameters specific tothe DHCP client host requesting, generally, information required by theclient host to participate on an IP network. DHCP also provides amechanism for allocation of IP addresses to client hosts.

A VoIP service network comprises terminals and local-area network (LAN)switches and/or hubs, which may be integrated to the terminalfunctionality. A VoIP service network may implement point-to-point andgroup/conference calls.

A problem arises when one or more of the VoIP servers are out ofservice. For instance, absence of the SIP server makes conventionalgroup calls impossible. Absence of the DHCP server is an even worseproblem, which would result in total loss of communication as terminalsare unable to resolve IP addresses. Use of static IP addresses isgenerally impractical in real-world networks with large numbers ofroaming terminals.

While the above description of problems of prior art networks relate toVoIP traffic, those skilled in the art will realize that similarproblems affect other types of communication, such as data traffic.

SUMMARY

An object of the invention is to improve fault-tolerance of existing IPservice networks. The object of the invention is achieved by terminalequipment, methods and software products as specified in the attachedindependent claims. The dependent claims and the following descriptionand drawings relate to specific embodiments and optional features of theinvention.

An aspect of the invention is an IP terminal according to claim 1.Specifically, the IP terminal according to the invention comprisescommunication means for communicating via an IP network. The IP terminalfurther comprises a processor and memory, wherein the memory contains anoperating software for the IP terminal and the processor is configuredto execute the operating software. The operating software comprises anormal mode logic for implementing a normal mode operation and arestricted mode logic for implementing a restricted mode operation. Thenormal mode logic comprises program code for initiating a call of afirst type. Calls of the first type mean calls that take place undercontrol of instructions from one or more dedicated servers. Therestricted mode logic comprises program code for collecting connectioninformation of other IP terminals and for initiating a call of a secondtype, which means calls taking place without instructions from the oneor more dedicated servers.

A terminal according to the invention is thus capable of normal modeoperation substantially in compliance with the above-identified RFCstandards. More particularly, the normal mode operation involves makinguse of the facilities provided by a conventional SIP server. Inrestricted mode operation, however, the terminal according to theinvention is capable of participating in point-to-point and/or groupcalls, both as an initiator and as an invited participant, without theSIP server. Terminals in the same LAN area are able to create arestricted mode IP network without using external servers, by connectingto one another via switches or hubs.

Thus the inventive terminal improves redundancy of IP networks becauseno single server is crucial. Instead any terminal can act as a temporarygroup call server and/or IP address handler. In order to keep the addedcomplexity of the terminals reasonable, group calls in restricted modemay be more restricted than conventional group calls, ie, group callsorganized by dedicated SIP servers. In the following, a group callorganized by a dedicated SIP server will be called a group call of afirst type, while a group call organized by a terminal, in absence ofthe dedicated SIP server, will be called a group call of a second type.Group calls of the second type may be more restricted than group callsof the first type, although this is not absolutely necessary. Forinstance, the processor of a typical single-user terminal may notpossess sufficient processing power for combining voices from severalgroup call participants, but second-type group calls may nevertheless beimplemented in half-duplex mode. Half-duplex mode means that only oneparticipant can speak (transmit) at a time. Accordingly, an embodimentof the terminal comprises means for allocating a transmission privilegeto one participating terminal at a time. For instance, the means forallocating a transmission privilege may comprise means for implementinga token-passing resource reservation scheme which may be implementedsuch that a virtual token, the possession of which indicates thetransmission privilege, is initially possessed by the terminalinitiating the group call. The initiating terminal receives a tokenrequest from another terminal and gives the token to the requestingterminal. While that terminal holds the token, the initiating terminalmay buffer any token request sent by other terminals. If the terminalholding the token holds it for a time exceeding some predeterminedlimit, the initiating terminal may force the token-holding terminal torelease the token. Further token requests from other terminals may beprioritised based on the length of time since those terminals last heldthe token. For example, a terminal that hasn't held the token for a longtime is prioritised higher than a terminal that held the token recently.Alternatively or additionally, the token requests from other terminalsmay be negatively prioritised based on the proportion of time theterminals have held the token. For instance, a terminal having held thetoken for 10% of the time is prioritised over another terminal havingheld the token for 30% of the time. As a further option, token requestsmay be prioritised based on the role assigned to the terminal. Roleswill be described in the following.

The terminal is preferably configured to output an audible and/or visualindication of the transmission privilege allocated to the terminal, suchas the possession of the token, because this is an indication to theterminal user that he/she can now speak.

Another optional feature for enhancing the functionality of the terminalis means for assigning a role to the terminal and means for originatinga call to another terminal and/or for responding to a call from anotherterminal, wherein the call is addressed on the basis of the roleassigned to the terminal that receives the call. The terminal may assignthe role in response to a user input. For instance, the user of aterminal being used by persons patrolling on route abc or watching gatexyz may use the terminal's user interface to create a role or to selecta pre-created role labelled “patrol route abc” or “watch gate xyz”. Abenefit of such role-based communication is that the terminals accordingto this embodiment and their users can communicate with other terminalswithout having to know the identity of the receiving terminal or itsuser.

An embodiment of the terminal according to the invention furthercomprises means for spontaneous mode transition from the normal modeoperation to the restricted mode operation in response to a detectionthat normal network service is not available. A benefit of thisembodiment is that no human action is required to detect the absence ofnormal network service. For instance the spontaneous transition torestricted mode operation can be triggered by a positive determinationthat a connection to the IP network and/or to the dedicated SIP serveris lost. Absence of the dedicated servers can be detected by sendingICMP echo messages to the server (“pinging”). It is beneficial toimplement a mode of operation wherein one terminal, for instance theterminal having the highest MAC address within the current subnetwork,performs the pinging and informs other terminals of the subnetwork orgroup as soon as a server is missing.

A conventional terminal accepts an address allocated by a dedicated DHCPserver. An embodiment of the terminal according to the invention furthercomprises means an IP address handler which replaces the dedicated DHCPserver and is configured to allocate an IP address to the terminalitself. The IP address handler may comprise a dynamic network topologyhandler which comprises program code for periodically broadcasting theterminal's own presence and for gathering presence information frompresence broadcasts of other terminals. Those skilled in the art willunderstand that the term topology will be used in a loose sense, and forthe purposes of the present invention, the topology of a subnetwork issufficiently defined by the terminals and network elements present inthe subnetwork and their IP addresses. In other words, topology of asubnetwork refers to the collection of IP address present in thesubnetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail bymeans of specific embodiments with reference to the attached drawings,in which

FIG. 1 schematically illustrates signalling relating to group calls in arestricted mode VoIP network;

FIG. 2 is a signalling diagram relating to hypothetical set of eventsduring group call establishment in a restricted mode VoIP network;

FIG. 3 shows a signalling diagram relating to hypothetical set of eventsduring push-to-talk reservation signalling in a restricted mode VoIPnetwork;

FIG. 4 illustrates signalling relating to presence broadcasting asutilized by a network topology handler according to an embodiment of theinvention;

FIG. 5 is a flowchart illustrating IP address handling by a terminalaccording to some embodiments of the invention;

FIG. 6 is a schematic block diagram of terminal hardware; and

FIG. 7 is a schematic block diagram of terminal software.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates signalling relating to group calls in arestricted mode VoIP network. As stated earlier, a dedicated SIP server(not shown) is responsible for group call establishment in normal (full)operation of a VoIP network, in accordance with RFC standards 2500,3261, 2543, 2327, 3264 3515 and 3265. The terminals and network elementsaccording to the present invention may be implemented such that innormal operation, the terminals and network elements operate in aconventional manner, and the departure from the prior art is onlyevident in restricted mode of the VoIP network wherein at least one keyfunctionality of the network is missing, as specified later inconnection with FIG. 5.

In restricted mode the terminals and network elements according to thepresent invention are capable of establishing group calls without adedicated SIP server and/or DHCP server. Instead a terminal that needsto initiate a conference call acts as a SIP server. In the following,the term “integrated SIP server” refers to the SIP server functionalityintegrated into the terminal. In order to keep system complexity withinreasonable limits, the integrated SIP server may be implemented in amore limited manner compared to the operation of a dedicated SIP server.For instance, group calls may be implemented in half duplex mode only,which means that group call participants do not transmit simultaneouslybut in sequence, after one another. In one representativeimplementation, the sequential (as opposed to simultaneous) transmissionmay be controlled by token passing, as will be described in more detailin connection with FIG. 3. For example, in one non-restrictiveimplementation, the terminal currently holding the token may transmitmedia as a UDP broadcast stream which is received by other participatingterminals.

FIG. 2 is a signalling diagram relating to hypothetical set of eventsduring group call establishment in a restricted mode VoIP network. Asstated earlier, restricted mode means that a dedicated SIP server is notavailable, and one of the terminals acts as a temporary SIP server. Inthe scenario shown in FIG. 2, terminal 21, labelled “Jack” after itsuser, acts as a temporary server in the setup phase of a group callwhich involves two other terminals 22 and 23, labelled “Phil” and “Eve”.In steps 2-101 and 2-102, the terminal 21 of Jack, acting as thetemporary server, sends group call invitation messages to terminals 22“Phil” and 23 “Eve”. In step 2-103, terminal 23 “Eve” sends an OKmessage to the server terminal 21, which sends an acknowledgment messageto terminal 23 “Eve” in step 2-104. Steps 2-105 and 2-106 are similar tosteps 2-103 and 2-104 but relate to control terminal 22 “Phil”. Thesignalling messages 2-101 through 2-106 shown in FIG. 2 can be similarto those used in conventional VoIP group call establishment, and the keydifference between the conventional VoIP group call and the presentinvention is that in restricted mode, wherein a dedicated SIP server isnot available, its functionality is implemented by one of the terminalswhich are to participate in the group call. Group call establishment maybe responsive to an instruction via the terminal's user interface. Theinstruction to set up a group call may be received via the terminal'skeypad, touch-sensitive display or via voice recognition, for example.

FIG. 3 shows a signalling diagram relating to hypothetical set of eventsduring talk permission reservation signalling in a restricted mode VoIPnetwork. The embodiment shown in FIG. 3 is an implementation of atoken-passing resource reservation.

In step 3-101, terminal 22 “Phil” sends a token request to the temporaryserver, namely terminal 21 “Jack”. In step 3-102, terminal 21 “Jack”gives the token to terminal 22 “Phil” which acknowledges the token instep 3-103. Terminal 22 “Phil” is now the only terminal of the groupwith the permission to talk. In two consecutive steps 3-104 and 3-105,terminal 23 “Eve” requests the token from the temporary server, namelyterminal 21 “Jack”, but terminal 21 cannot give the token because it ispresently assigned to terminal 22 “Phil”. In step 3-106 terminal 22“Phil” releases the token and the token release is acknowledged byterminal 21 “Jack” in step 3-107. In step 3-108 terminal 23 “Eve” againrequests the token and receives it in step 3-109. In step 3-110 terminal23 “Eve” acknowledges the token.

In summary, FIG. 3 shows signalling steps among one temporary server andtwo other participating terminals, one of which obtains the token andreleases it after which the other terminal can obtain the token. Abenefit of the token passing technique is that it is relative easy toimplement, considering the fact that a dedicated server's functionalityis temporarily handled by one of the participating terminals. Aconsequence of the simplicity is the fact that only half-duplexoperation is supported, and more elaborate embodiments of the inventionmay have more processing power and be able to implement audio-summingschemes, thus supporting full duplex operation.

FIGS. 1 to 3 have been described in connection with VoIP traffic, butthe considerations are applicable to other types of traffic, such asdata traffic.

FIG. 4 illustrates signalling relating to presence broadcasting asutilized by a network topology handler according to an embodiment of theinvention. By definition, in a restricted mode network one or morenetwork elements or inter-element connections are out of operation butthe remaining network elements and terminals have no a priori knowledgeconcerning which element or connection is malfunctioning or missing.Accordingly, the terminals of the inventive IP network implement adynamic network topology handler as schematically shown in FIG. 4. Eachterminal periodically broadcasts messages about itself. In theillustrative example shown in FIG. 4, terminal 44 broadcasts itsidentity information which may include the role name of the terminal(“Mr Smith”), IP address (herein: “10.1.2.2”) and MAC address (herein“01:02:03:04:05:06”). On receiving this information, the other terminals45 and 46 add the identity information of terminal 44 (labelled “MrSmith”) into their network topology data. In the context of the presentinvention, the term “topology” has a broad definition and meanscollected connection information on other terminals, ie, informationspecifying how to connect calls to the other terminals. For example,such connection information can be include the IP address, MAC addressand/or role name of the terminals. In order to keep the network handlingdynamic and adaptive, it is beneficial to implement a feature wherein,if the broadcasted identity information of a certain unit is notreceived for a predetermined time, the entry for that unit is deleted.Accordingly, each unit should periodically broadcast its identityinformation and the predetermined time for deleting identity informationof terminals whose identity information broadcasts have not beenreceived is longer than the period between successive identityinformation broadcasts. It is also beneficial to send presencebroadcasts regularly with a predefined period, even when all roleinformation is not available for the terminal. This allows otherterminals to determine IP addresses being used by others.

In addition, each terminal comprises a fault-state/normal-state handlerwhich is coupled with the network topology handler whose operation wasdiscussed in connection with FIG. 4. The fault-state/normal-statehandler determines whether or not a connection to a SIP server isavailable. The connection to the SIP server may be tested by polling theSIP server, for example. Each terminal in the same broadcast area knowsthe (Ethernet) MAC address of every other unit. In an illustrativeimplementation, the unit having the highest MAC address regularly pollsthe SIP server. If the SIP server fails to respond to the poll, thepolling unit may perform another polling attempt. If the SIP serverfails to respond to the polling attempt(s), the unit performing thepolling may broadcast messages to the network, indicating that theconnection to the SIP server is lost. The polling unit may keep pollingthe SIP server. In one illustrative implementation, each unit thatreceives the broadcast message about the unavailability of the SIPserver acts as if it had itself detected the unavailability of the SIPserver and enters restricted mode. On the other hand, if the unit havingthe highest MAC address disappears from the network, the one having thehighest MAC address among the remaining units will begin to poll the SIPserver. If the SIP server's operational status is restored, the pollingunit broadcasts this information to the network, after which the unitsresume normal mode operation.

Another fault state detection relates to the DHCP server. As is known tothose skilled in the art, IP networks employ Dynamic Host ConfigurationProtocol, or DHCP, servers which normally allocate IP addresses to thenetwork terminals. It is possible to operate a network or subnetworkwith static IP addresses, but such a mode of operation is impractical inreal-world networks. A normal DHCP server should be configured such thatif a terminal's connection to the server is lost, there is adequatelease time left for IP addresses so that problem can be fixed until thelease time expires. For instance, if the lease time of addresses is 48hours and the IP addresses are reallocated at 24-hour intervals, therewill always be at least 24 hours left to fix problems relating to theDHCP server.

According to an implementation of the invention, if the DHCP server isout of operation long enough for the lease time of the IP addresses toexpire, the terminals continue use of the latest IP address allocated tothem.

In an alternative implementation, after a restart, for example, when theterminals are connected to a network, they generate their own IPaddresses. A unit may generate its own IP addresses as follows. IPaddress generation loosely follows standard RFC 3927, with the majorexception that this altered implementation adapts to an IP address spacealready being used in the LAN, whenever possible. First the unitobserves the network and gathers IP addresses existing in the networkfor a predetermined period of time, after which it selects an availableIP address itself. IP address selection is preferably performed by usingIP packets which are certainly known to contain a correct IP address,such as precedence broadcast messages mentioned elsewhere in this patentspecification. The selected IP address is preferably between the highestand lowest IP addresses gathered during the observation period. In somesituations this may not be possible. For instance, there may not be anygaps between the observed IP addresses. In such cases the unit mayselect the first IP address higher than the highest observed IP address.Alternatively, the first IP address lower than the lowest observed IPaddress may be selected for address generation. The unit carries out anAddress Resolution Protocol (ARP) request, and if it receives noresponse to the ARP request, the unit begins to use the selected IPaddress. If a response is received, this means that the selected IPaddress is reserved (although it was not used for broadcasting duringthe observation period). Accordingly, the unit selects the next higher(or lower) IP address and retries the ARP request until an available IPaddress is determined.

FIG. 5 is a flowchart illustrating IP address handling by terminalaccording to some embodiments of the invention. Reference numeral 5-00does not denote some unambiguously identified state but rather denotesany state in the terminal's continuous operation, based on theterminal's pre-existing IP address, as typically defined in theaforementioned RFC standards. Reference numeral 5-10 denotes a testwherein a negative result of IP address collision detection test causesthe terminal to remain in the previous state, whatever it was, while apositive result of IP address collision detection test causes a transferto step 5-20, which is the initial state of the inventive restrictedmode which is the topic of the present invention. It is also possible toreach step 5-20 via power-on operation. There may be alternative oradditional diagnostic tests whose positive result causes a transition tothe restricted mode beginning from step 5-20.

From step 5-20, the terminal proceeds to step 5-22 to check if DHCPservice is available. If yes, the terminal obtains the IP address fromthe DHCP server in step 5-30 after which it proceeds to normal idlestate operation in step 5-32. The terminal remains in normal idle state5-32 until two consecutive checks 5-34 and 5-36 indicate, respectively,that DHCP service has disappeared and the lease time of the terminal'sIP address has expired, in which case the terminal proceeds to normalno-DHCP idle state, denoted by reference sign 5-38. The terminal remainsin normal no-DHCP idle state 5-38 until a check in step 5-40 indicatesthat DHCP service is available, after which the terminal proceeds tore-boot state 5-20 of the restricted mode.

On the other hand, if DHCP service is not available in step 5-22, theterminal proceeds to step 5-50 to gather the network topologyinformation (cf. FIG. 4). In step 5-52 the terminal selects one of twobranches, depending on whether or not sufficient information on thenetwork topology has been obtained. In one illustrative implementationthe terminal obtains such network topology information by listening tothe network traffic and collecting IP addresses of senders andrecipients of data packets for a predetermined time. If sufficientnetwork topology information has not been obtained, the terminal takesbranch 5-54 through 5-58, wherein the terminal generates a random IPaddress in some predetermined range (an exemplary range 169.254.0.2through 169.254.254.254 is shown in FIG. 5) and sends an ARP request forthe randomly-generated IP address. After that, in step 5-58 the terminalchecks if a reply to the ARP request is received. If yes, the terminalreturns to step 5-54.

In step 5-52, if the terminal has obtained sufficient network topologyinformation, it takes branch 5-60 through 5-64, wherein the terminalselects an IP address and sends an ARP request to the selected IPaddress. After that, in step 5-64 the terminal checks if a reply to theARP request is received. If yes, the terminal returns to step 5-60.

In steps 5-58 and 5-64, if the ARP request fails to result in an ARPreply, this means that the selected or randomly generated IP addressappears to be available, and the terminal proceeds to normal no-DHCPidle state 5-38.

FIG. 6 is a schematic block diagram of terminal hardware, generallydenoted by reference numeral 600. The terminal hardware 600 comprises acentral processing unit (CPU) 602, memory 604, reception/transmissioncircuitry (RX, TX) 606 which comprises a reception circuitry 608 andtransmission circuitry 610. The reception/transmission circuitry 606provides operational coupling of the terminal 600 with other terminals,generally denoted by reference numeral 630. The terminals 600, 630 arecoupled via an IP network 622. In the example shown in FIG. 6, theterminal 600 is coupled to the IP network 622 via a local-area network620.

The terminal hardware 600 further comprises a user interface circuitry612 whose main sections are an input circuitry 614 and output circuitry616. The input circuitry 614 comprises a microphone for voice input,dialing means, such as a keypad or touch-sensitive display and,optionally, a conveniently placed push-to-talk button for restrictedmode operation (half duplex). For instance, the push-to-talk button maybe positioned such that it is conveniently operated by the user's thumb.At this level of generalization, all elements 600 through 630 can beconventional as used in the relevant art.

The central processing unit (CPU) 602 executes the terminal softwarewhich is stored in the memory 604. The software is described in moredetail in connection with FIG. 7.

FIG. 7 is a schematic block diagram of terminal software. The softwareis contained in the terminal's memory 604 from where the centralprocessor 602 retrieves program sections as required. The reader asassumed to be familiar with the operation of a conventional IP terminaland only the novel elements of the present invention are described indetail and a detailed description of such conventional functionality isomitted. In addition to software implementing conventional IP terminalfunctionality, the terminal software, generally denoted by referencenumeral 700, comprises logic sections 702 through 704 for implementinggroup calls in restricted mode in which the one or more network servers,such as a SIP and/or DHCP server, are absent.

In restricted mode operation, when the terminal receives an indicationfrom its user that the terminal should establish a group call, theserver functionality is provided by a group call signalling handler 702,which is typically implemented as a client/server module. The serverpart is operational in the unit that initiates the group call, while theclient part is operational in other participants of the group call. Thegroup call signalling handler 702 queries a subnetwork topology handler704 for available terminals and invites such terminals to the groupcall. The invited terminals will act as clients. The group callsignalling handler 702 regularly queries the subnetwork topology handler704 for any change in the terminals present in the subnetwork. Anynewly-added terminals may be added to join an existing group call.Conversely, any terminal detected as disappeared by the subnetworktopology handler 704, are removed from the list of group callparticipants. Operation of the subnetwork topology handler 704 wasdescribed earlier, in connection with FIG. 4.

An RTP stream manager 706 controls streaming of audio packets accordingto information received from group call signalling handler 702. Forinstance, the RTP stream manager 706 may configure and start an RTPstream containing audio information, while the group call signallinghandler 702, acting as a client, has received the token. In downlinkdirection (data to speech), propagation time differences between UDPdata packets may be reduced or eliminated by a downlink jitter buffer710.

In a typical implementation, all inter-process interfacing in theterminal may take place via FIFO message buffers.

It is readily apparent to a person skilled in the art that, as thetechnology advances, the inventive concept can be implemented in variousways. The invention and its embodiments are not limited to the examplesdescribed above but may vary within the scope of the claims.

1. An Internet Protocol (IP) terminal, comprising: means forcommunicating via an IP network; a processor; and a memory, wherein thememory contains operating software for the IP terminal and the processoris configured to execute the operating software, wherein the operatingsoftware comprises a normal mode logic for implementing a normal modeoperation and a restricted mode logic for implementing a restricted modeoperation, the normal mode logic comprising program code for initiatinga call of a first type under control of instructions from one or morededicated servers, and the restricted mode logic comprising program codefor collecting connection information of other IP terminals and forinitiating a call of a second type without instructions from the one ormore dedicated servers.
 2. An IP terminal according to claim 1, furthercomprising means for spontaneous mode transition from the normal modeoperation to the restricted mode operation in response to at least oneof the following: a positive determination that a connection to the IPnetwork is lost and a positive determination that a connection to atleast one of the one or more dedicated servers has failed after a mostrecent reset of the IP terminal.
 3. An IP terminal according to claim 2,further comprising means for spontaneous mode transition from therestricted mode operation to the normal mode operation in response to apositive determination that a connection to at least one of the one ormore dedicated servers has been restored after the IP terminal's latestmode transition to the restricted mode operation.
 4. An IP terminalaccording to claim 1, further comprising: means for assigning a role tothe IP terminal, in response to a user input; and means for originatinga call to another IP terminal and/or for responding to a call fromanother IP terminal, wherein the call is addressed on the basis of therole assigned to the IP terminal receiving the call, whereby the IPterminal can communicate with the other IP terminal without having toknow an identity of the receiving IP terminal or its user.
 5. An IPterminal according to claim 4, further comprising: means for assigning arole to the IP terminal, in response to a user input; means forresponding to an incoming call from another IP terminal, whereinsignalling relating to the incoming call indicates the role assigned tothe IP terminal originating the call; and means for assigning a priorityto the incoming call based on the role assigned to the IP terminaloriginating the call.
 6. An IP terminal according to claim 1, whereinthe normal mode logic comprises program code for accepting an IP addressallocated by a dedicated server and the restricted mode logic comprisesan IP address handler configured to allocate an IP address to the IPterminal itself.
 7. An IP terminal according to claim 6, wherein the IPaddress handler comprises program code for periodically broadcasting theIP terminal's own presence.
 8. An IP terminal according to claim 1,wherein the restricted mode logic comprises program code for initiatingand/or controlling a group call of the second type and the IP terminalis configured to participate in the group call of the second type and toinvite at least two other IP terminals to the group call of the secondtype.
 9. An IP terminal according to claim 8, wherein the IP terminalcomprises means for allocating a transmission privilege to oneparticipating terminal at a time.
 10. An IP terminal according to claim9, wherein the means for allocating a transmission privilege comprisesmeans for implementing a token-passing resource reservation scheme. 11.An IP terminal according to claim 9, further comprising means foroutputting an audible and/or visual indication of the transmissionprivilege allocated to the IP terminal.
 12. An operating method for anInternet Protocol (IP) terminal, which comprises a processor and memory,wherein the memory contains operating software for the IP terminal andthe processor is configured to execute the operating software, thuscarrying out the operating method for the IP terminal, the methodcomprising: communicating via an IP network in a normal mode, whereincommunicating in the normal mode comprises initiating a call of a firsttype under control of instructions from one or more dedicated servers;and communicating via the IP network in a restricted mode, whereincommunicating in the restricted mode comprises collecting connectioninformation of other IP terminals and for initiating a call of a secondtype without instructions from the one or more dedicated servers.
 13. Acomputer-readable storage medium comprising instructions that, whenexecuted by a processing device, cause the processing device to performoperating software for an Internet Protocol (IP) terminal by:communicating via an IP network in a normal mode, wherein communicatingin the normal mode comprises initiating a call of a first type undercontrol of instructions from one or more dedicated servers; andcommunicating via the IP network in a restricted mode, whereincommunicating in the restricted mode comprises collecting connectioninformation of other IP terminals and for initiating a call of a secondtype without instructions from the one or more dedicated servers.